Retire SR-IOV, NIC tuning and watchdog scripts to unused/

Checked against the live host. None of these are in service:

- sriov-enp4s0.service and set-ixgbe-vfmacs-enp4s0.sh target enp4s0f1np1,
  which no longer exists; sriov_numvfs is 0 on every port and the unit was
  never installed.
- nic-performance-tweaks.service was never installed either, though its
  interfaces do still exist, so it would work if reinstated.
- vm-watchdog.sh watches VMID 2801 "pfsense", which no longer exists.
- lxc-watchdog.sh still carries the upstream example targets: CT 100 and
  CT 101 on 192.168.31.0/24, a subnet that is not in use.

Neither watchdog is scheduled; there is no root crontab.

The watchdog config moves to unused/conf/ alongside them, so the scripts
resolve it unchanged from their new location.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

unused/conf/watchdog.conf.example

@@ -0,0 +1,11 @@
+# Alert destination for vm-watchdog.sh and lxc-watchdog.sh.
+#
+# Copy to conf/watchdog.conf next to the scripts:
+#   cp conf/watchdog.conf.example conf/watchdog.conf
+#
+# The scripts look for it relative to their own location, so this works both in a
+# checkout and deployed at /opt/ergosteur. Override with WATCHDOG_CONFIG.
+#
+# If the file is absent the watchdogs still run and log; they just skip the email.
+
+WATCHDOG_EMAIL=alerts@example.com